1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method and, more particularly, to an image processing apparatus for suppressing color crosstalk in an image sensed by an image sensor, and a control method thereof.
2. Description of the Related Art
An image sensor mounted in an image-capturing apparatus such as a digital camera has an arrangement in which pixels each having a photoelectric conversion element are arrayed in a matrix. The image sensor converts an object image formed by an optical system into an electrical signal for each pixel. A problem in the image sensor having this arrangement is “color crosstalk” generated when obliquely incident light enters not only a predetermined pixel but also an adjacent pixel.
The degree of color crosstalk changes depending on the pixel color (color of a color filter arranged in a pixel). Color crosstalk becomes large in a pixel of another color adjacent to a color filter of a high-transmittance color (generally green). For example, in an image sensor having color filters of a general Bayer array, color crosstalk caused by light which obliquely enters a green pixel out of red, green, and blue pixels, passes through the green color filter, and then enters an adjacent blue or red pixel becomes relatively large. The color crosstalk relatively increases the sensitivity of a blue or red pixel adjacent to a green pixel with respect to the green wavelength, and relatively decreases that of a green pixel.
As a technique for canceling color crosstalk, Japanese Patent Laid-Open No. 10-271519 discloses a technique of storing a correction coefficient determined based on the color crosstalk component, and correcting, using the correction coefficient, a color crosstalk component contained in an image signal output from an image sensor.
Japanese Patent Laid-Open No. 2008-263492 discloses a technique of estimating the change amount of the F-number upon a zoom lens focal length (angle of view) change operation, and controlling the shutter speed and frame rate, thereby preventing overexposure or underexposure when a zoom operation is done during moving image capturing.
Japanese Patent Laid-Open No. 11-194390 discloses a technique of enabling fine light quantity adjustment by a combination of stop driving using a stepping motor and electronic shutter speed control in a video camera or the like without increasing the number of steps of the stop.
A method of preparing in advance a correction coefficient for correcting generated color crosstalk and correcting an image signal, as disclosed in Japanese Patent Laid-Open No. 10-271519, corrects an image signal by applying, to each color pixel, a gain for correcting a relative sensitivity difference caused by color crosstalk. When color crosstalk is large (sensitivity difference is large), a large gain needs to be applied. This may decrease the S/N ratio or degrade the image quality owing to overcorrection and undercorrection. When different gains are applied for respective, red, green, and blue pixel colors, the luminance value changes upon a change of a luminance composition ratio given by red:green:blue=3:6:1. Color crosstalk correction changes the exposure of an image to be captured.
For this reason, if the degree of color crosstalk changes owing to a change of the aperture value, the white balance coefficient needs to be obtained again. However, the techniques disclosed in Japanese Patent Laid-Open Nos. 2008-263492 and 11-194390 cannot correct color cast caused by color crosstalk for a frame image which undergoes white balance processing during a period until a white balance coefficient corresponding to a changed aperture value is obtained when the aperture value changes during moving image capturing.